This invention relates to metal/air cells in general and more particularly to an improved hybrid electrode for use in such cells. Hybrid electrodes are known and are disclosed for example in German Offenlegungsschrift 1,921,157. A characteristic of this type of electrode is that it contains in one electrode a catalyst for gas separation or precipitation along with a catalyst for gas dissolution, with the catalysts arranged in two separate layers. In such a hybrid electrode a hydrophilic layer on the electrolyte side contains the catalyst for gas separation and a hydrophobic layer on the gas side contains the catalyst for gas dissolution. Nickel or graphite can be used as the catalysts for gas separation. Catalysts for gas dissolution may be carbon, silver, silver-impregnated carbon along with combinations of carbon with nickel oxide and cobalt oxide, carbon with cobalt oxide and aluminum oxide or carbon with magnesium oxide.
The significant advantage of a hybrid electrode is that only the catalysts used for the gas separation and the frame material that may be present need be corrosion-resistant at the potential of the gas separation. This results from the concept that the hydrophilic layer on the electrolyte side allows the flow lines to penetrate only negligibly into the hydrophobic layer on the gas side so that particles of the layer on the gas side which have contact with the metal electrode through the electrolytic liquid are at the normal at rest potential even during gas separation and are thus protected against corrosion. As a result the range of frame and catalyst materials which can be used on the gas side is substantially increased.
Hybrid electrodes for the dissolution and separation of oxygen for metal/air cells and which consist of a hydrophilic nickel layer on the electrolyte side used for O.sub.2 separation, a hydrophobic carbon layer for the O.sub.2 dissolution and a hydrophobic plastic layer on the gas side which prevents the electrolyte from escaping are known and are disclosed in Siemens Forschungsund Entwicklungsberichte, vol. 1, No. 2/72, page 221 to 226. In an arrangement such as this a metal screen for taking off current can be embedded in the hydrophobic carbon layer.
Electrodes of this nature have been found to be advantageous in comparison with other known electrodes. However with a cyclic load of 6 hours of O.sub.2 separation and then 6 hours of 0.sub.2 dissolution at a current density of 30 mA/cm.sup.2 and a potential of -200 to -300 mV with respect to an Hg/HgO/6 m KOH reference electrode, they have a life of only 50 to 60 cycles. The limited life of these electrodes results almost exclusively from the lack of mechanical cohesion between the individual layers. Plastic contained within the carbon layer to make this layer hydrophobic is also supposed to take care of the mechanical stability of this layer and bring about adhesion of the individual layers to each other. However it has been found that even with high percentages of plastic, such as plastic in the range of 40% by weight, the adhesion of the carbon layer to the nickel layer and the adhesion of the plastic layer to the carbon layer along with the mechanical stability of the carbon layer itself cannot be assured over extended periods of time. The mechanical stresses result particularly from the oxygen which is developed in the charging process at the boundary between the nickel and carbon layer and which results in the carbon layer being blown off from the nickel layer. Mechanical stresses also occur at the boundary between the carbon layer and the plastic layer probably because of the pressure exerted by the electrolytic liquid.
In view of these difficiencies, the need for an improved hybrid electrode which has increased mechanical supports so that its useful life can be extended becomes evident.